In order to prevent occurrence of a burr on the product or otherwise improve the quality of metal stamping or to improve the durability of the stamping tool, such as a punch and a die, it is essential to reduce the friction at the friction interface during metal stamping. The workpiece material (metal material) has irregularities on the surface to a greater or lesser extent. The lubricant (lubricating oil) applied to the material before metal stamping is accumulated in the recesses forming the irregularities. If there is no clearance between the stamping tool and the material into which the lubricant can escape when the stamping tool and the material come into contact with each other, the lubricant is mechanically sealed in the recesses and introduced to the friction interface. The introduction mechanism is expected to work in all plastic working.
It has been confirmed that, if the surface of the material or stamping tool is previously roughened by acid corrosion, shot peening, sand blast or the like to make the use of the effect of the lubricant, the frictional force decreases, and galling or seizing can be prevented. The part in which the lubricant is sealed is referred to as a micro pool in the field of plastic working. In working, high hydrostatic pressure builds up in the lubricant in the micro pool and makes a contribution to the pressure at the worked surface. In this state, wear of the stamping tool surface is reduced. Therefore, it is important to condition the stamping tool surface to have a roughness with which high hydrostatic pressure occurs in the lubricant in the micro pool.
For example, Patent Literature 1 discloses an art that achieves high stampability by conditioning the surface of an alloy material by heat treatment to form micro pools. Patent Literature 2 discloses an art that improves the working precision and elongates the life of the stamping tool by spraying hard particles onto the stamping tool surface to harden the stamping tool surface and form micro pools.    Patent Literature 1: Japanese Patent Application Laid-Open No. H07-18403    Patent Literature 2: Japanese Patent Application Laid-Open No. 2008-55426